In IVP Paul & letters I read:
"Paul uses the word ouranos twenty-one times, twelve of these in the singular and nine in the plural. It is frequently assumed that the plural uses reflect the Hebrew sÎaœmayim but…"
The transliteration sÎaœmayim looks very wrong.
But putting "sÎaœmayim" into Search All brings up
"The most common words for heaven in the OT are the Hebrew s¥aœmayˆîm or the…" in IVP Jesus & gospels.
[It doesn't actually look like that but it doesn't paste properly here].
I think the first reference must be coded wrong or should be in a different font?